The hematopoietic system originates from a small population of self-renewing hematopoietic stem cells (HSCs) that in turn derive from hemangioblasts, common precursors of blood and blood vessels. Extensive studies have identified key transcription factors, such as the basic helix loop helix (bLH) SCL/TAL1 and GATA factors, which play critical roles in the successive steps of differentiation of HSCs into the erythroid, myeloid and lymphoid lineages. Despite these advances, the nature of the factors that commit the hemangioblast to blood and endothelial cells are obscure and the transcriptional networks operative in HSC self-renewal and lineage commitment remain incomplete. In preliminary studies, we have identified the Kruppel-like zinc finger transcription factor ZBP-89 as a master regulator of early development of FLK1+ mesoderm into blood and blood vessels. Forced expression of ZBP-89 increased expression of primitive and definitive hematopoiesis in zebrafish and in mouse embryonic stem cells (ESCs)/embryonic body (EB) cultures, but reduced axial and intersomitic blood vessel formation in zebrafish embryos and the endothelial replating potential and sprouting angiogenesis in mouse EB cultures. Knockdown of ZBP-89 in zebrafish or in mouse ESCs resulted in a dramatic reduction in primitive and definitive hematopoietic markers (e.g. SCL and GATA factors), but an increase in endothelial lineage markers, distinguishing the mechanism of action of ZBP-89 from all other known transcription factors regulating hematovascular development. Further, we find that mouse pubs homozygous for a hypomorphic ZBP-89 mutant allele die perinatally and exhibit a marked reduction in circulating red blood cells, but an increase in mature myeloid cells, reflecting an additional critical role for ZBP-89 in fetal hematopoiesis, perhaps at the level of the bipotential Common Myeloid Progenitor (CMP) stem cell, which gives rise to both the erythroid and myeloid lineages. The effects of ZBP-89 modulation on adult hematopoiesis and the underlying transcriptional networks involved in its action at the level of hemangioblasts and CMPs are unexplored. In this application, we propose to assess the consequences of loss of ZBP-89 on fetal and adult hematopoiesis and vascular development in vivo (Aim 1), determine the effects of its ectopic expression on blood and vessel lineage development in adult mice and zebrafish (Aim 2), and elucidate the mechanism(s) underlying ZBP-89- mediated hematopoietic lineage commitment. (Aim 3). Genetic, biochemical, proteomics, genomics, stem cell cultures and bone marrow transplantation in mouse and zebrafish will be utilized.